Many dyes, as well as pigments, are prepared by a two-step process of diazotization of an aromatic amino compound followed by coupling of the product. In certain applications, such as when the dye is used in an aqueous ink for inkjet printers, it is desirable to exclude, so far as possible, any impurities such as salts. Salt impurities in aqueous inkjet printer inks tend to promote the corrosion of the metal parts of an inkjet printer. In addition, salt impurities in certain aqueous ink compositions may tend to destabilize the ink components, causing the ink components to precipitate.
Hinson et al., U.S. Pat. No. 4,617,381 discloses a process for preparation of Cl Direct Yellow 11 in which the tetrasodium salt is acidified with sulfuric acid to a pH of less than 2.5, then heated. The salt is then neutralized to a pH of 6.0 to 7.5 with an alkanolamine to precipitate the alkanolamine complex. The free acid form is disclosed as having extremely poor filtration characteristics, as the salts formed by neutralization with either sulfuric or hydrochloric acid form a viscous slime that could not be filtered. The alkanolamine complex is reported to have improved stability.
Ono et al., U.S. Pat. No. 5,366,543 discloses preparation of a diazo dye salted with a substituted amine having at least one group of 6 to 12 carbon atoms. Two moles of sec-butylaniline are diazotized and then coupled with 1 mole of di-J acid (bis-(5,5'-dihydroxy-2,2'-naphythyl)amine-7,7'-disulfonic acid) or a similar coupling compound. The water-soluble dye product is salted with an amine having at least one group with 6-12 carbon atoms by mixing an excess of the amine with the water-soluble dye in aqueous solution for 2-5 hours in acidic or neutral atmosphere. When the amine is water-insoluble or has low solubility in water, the salt-formation step is carried out in an organic solvent such as an alcohol followed by addition of the solution into an aqueous acid solution, one example being aqueous acetic acid, to precipitated out the amine-salted dye. The precipitated dye is then filtered and rinsed with water.
Weberndoerfer et al., U.S. Pat. No. 4,560,745 discloses a process for preparing sulfonic acid functional dyes having low electrolyte content. An aqueous solution or suspension of the dyes is mixed with a water insoluble amine having 12 to 40 carbon atoms to separate the dye into a lipophilic phase; bringing the pH to less than 5 with an acid, for example sulfuric acid, and mixing the batch thoroughly, separating the phases, and mixing the lipophilic dye phase with a water soluble base and water to produce an aqueous solution of the dye.
Bermes et al., U.S. Pat. No. 5,431,723 discloses a dye preparation that is essentially free of foreign salts. The dye is prepared by diazotizing 4,4'-diaminostilbene-2,2'-disulfonic acid and then coupling with 1-hydroxy-7-aminonaphthalene-3-sulfonic acid in alkaline medium. The coupled product is acidified with hydrochloric or sulfuric acid to precipitate the product dye. The precipitated product is filtered, washed with water or dilute hydrochloric acid, and dried.
While these references generally discuss purification of a dye product, it would be advantageous to have an improved method of producing an amino sulfonic acid compound-based dye by using a purified starting product. Some of the advantages to purifying the aromatic amino sulfonic acid starting material rather than the finished dye are that (1) the dye is often a form that is difficult to filter and wash, such as a tar-like lake; (2) there is less material that must be purified for the amino- and sulfonic acid-containing starting material as compared to the coupled dye; (3) material loss in the dye purification step is more expensive; and (4) there is no organic in the waste stream that would need more expensive disposal means.